Humanitarian groups have reported spotting dozens of boats carrying migrants off the coast of the Libyan port city of Sabratha.

Ships manned by humanitarian organizations, the Italian navy and coast guard helped rescue about 4,500 boat migrants on Thursday as calm seas returned to the Mediterranean, prompting a surge in departures from North Africa.

According to the International Organization for Migration, IOM, the controversial agreement between Turkey and the E.U. to stop migrant departures for the Greek islands has reduced boat arrivals by 98 percent during the first five months of the year from the same period of 2015.

But arrivals in Italy continue at about the same clip as last year, and the deadly central Mediterranean route has already claimed 2,438 lives, IOM said.

Italy has been on the front line of Europe's worst immigration crisis since World War II and now in its third year. More than 320,000 boat migrants came to Italy from North Africa in 2014-15.

As of Wednesday, 56,328 boat migrants had been brought to Italy in 2016, a 5.5 percent decrease on the same period of last year, according to the Interior Ministry, with Nigerians, Eritreans and Gambians the top three nationalities among migrants.